Since we haven't really seen a lot of haki fighters go up against each other, what we can do at most is speculate. And the simplest way to do so would be to judge by their reputation. And if I
were to do so my top picks would be the following: Roger, Rayleigh, Whitebeard and presumably Shanks and Dragon. Luffy does not belong here yet, but he most certainly will towards the end of the
We don't anything about his CoA so I won't mention that, but his CoO was vaguely mentioned by Rayleigh. "He had the ability the hear the voice of everything", now that could indicate on some sort of DF or anything similar, but judging by Rayleigh's explanation of Haki, it most likely was reference to his CoO. And if Rayliegh, someone who is toughht and assumed to be a master of all types of haki, is impressed by it then it's likely that it's also one of the strongest we've seen so far.
All these characters have the CoC, or at least are strongly hinted to have it, namely Dragon and Roger. Roger's haki would obviously be the strongest one of them if you were to grade them as such, which is a speculation mostly built on, as I said, his reputation and ambition. His CoC are most certainly one of the most powerful CoC seeing as the CoC has been reference being a power that most powerful individuals have in the NW. And since he's supposed to be one of the strongest, one easily draws the conclusion that his CoC will follow accordingly. Loosely based I know, but since we don't have much to go on, it's the best that we can draw that conclusion on right now.